Daylight saving time explained

Contrary to popular belief, no federal rule mandates that states or territories observe daylight saving time.

The idea of daylight saving time was introduced nationwide in 1918, when Congress formally approved the time zones the U.S. railroad industry had been following since 1883, according to National Geographic.

Part of the 1918 law also legislated for the observance of daylight saving time nationwide, but that section was repealed and the matter left to local jurisdictions a year later in 1919.

Daylight saving time was observed nationally again during World War II, but not uniformly practiced until 1966, when start and end dates were standardized. States could still opt out if they wished.

Most U.S. residents now set their clocks one hour forward in the spring and one hour back in the fall.

But people in Hawaii and most of Arizona-- along with Puerto Rico, American Samoa and the Virgin Islands -- will do nothing come Saturday. Those locales never deviate from standard time.

Daylight saving time is meant to decrease the amount of daylight in the morning hours, so that more daylight is available in the evening.

In 1986, the start date was moved from the last Sunday in April to the first Sunday in April.

In 2005, Congress pushed the start of daylight saving time forward three weeks to the second Sunday in March, rather than the first Sunday in April, and moved the end back one week to the first Sunday in November.

The changes went into effect in 2007 and were meant to save electricity, although a U.S. Department of Energy study estimated a savings of less than 1 percent of the country's annual energy consumption.

Among several time zone oddities, Newfoundland, Canada, has its own time zone in Canada and North America which is 30 minutes behind Eastern Standard Time. Alaska also has its own time zone.